Method of Separating Off Nitrogen from Liquefied Natural Gas

ABSTRACT

A method of separating off an N 2 -rich fraction from a liquefied hydrocarbon-rich fraction is described, in particular from liquefied natural gas, wherein the liquefied hydrocarbon-rich fraction, after its liquefaction and subcooling, is fed to a stripping column which serves for separating off the N 2 -rich fraction. 
     According to the invention, a first substream of the liquefied hydrocarbon-rich fraction ( 2, 2′ ) is applied to the stripping column (S) as reflux, whereas a second substream of the liquefied hydrocarbon-rich fraction ( 3, 3′ ) is fed to the bottom of the stripping column (S), wherein the second substream of the liquefied hydrocarbon-rich fraction ( 3, 3′ ) has a higher temperature than the first substream of the liquefied hydrocarbon-rich fraction ( 2, 2′ ).

The invention relates to a method of separating off an N₂-rich fraction from a liquefied hydrocarbon-rich fraction, in particular from liquefied natural gas, wherein the liquefied hydrocarbon-rich fraction, after its liquefaction and subcooling, is fed to a stripping column which serves for separating off the N₂-rich fraction.

Methods of the type in question for separating off an N₂-rich fraction from a liquefied hydrocarbon-rich fraction are used, in particular, in the liquefaction of natural gas. The liquefied natural gas (LNG) obtained by means of the prior art liquefaction method typically is allowed only to have a nitrogen content of a maximum of 1% by volume. If the nitrogen content is above this threshold value, generally separating off the impermissible nitrogen amount from the liquefied natural gas is required.

This nitrogen separation customarily proceeds via a pressure expansion of the natural gas which was liquefied under pressure, wherein via a suitable choice, with respect to temperature and pressure of the liquefied natural gas, of the initial state before expansion and of the pressure after expansion, a targeted outgassing of the liquefied natural gas is achieved. The nitrogen which is unwanted in the liquid phase, this is the wanted LNG product, is removed via the resultant gas phase.

In this procedure, however, there is the problem that, depending on the boiling equilibrium, the amount of methane outgassing together with the nitrogen to be removed is undesirably high. To avoid this disadvantage, methods of the type in question for separating off nitrogen from liquefied natural gas are used, in which, by providing a stripping column, nitrogen can be depleted more selectively, and simultaneously, the unwanted losses of methane to the nitrogen-rich fraction to be depleted can be decreased. Such a method of the type in question for separating off a nitrogen-rich fraction from a liquefied hydrocarbon-rich fraction may be taken, for example, from U.S. Pat. No. 5,893 274.

In the abovementioned procedure, the liquified hydrocarbon-rich fraction, after its expansion, is delivered in two phases to the top of the stripping column. The bottom of the stripping column is heated with the liquefied hydrocarbon-rich fraction, before its expansion, in indirect heat exchange via a reboiler. The liquid fraction produced in the bottom of the stripping column is the desired LNG product, whereas the nitrogen-rich gas fraction taken off at the top of the stripping column is generally then used as combustion gas.

However, it is disadvantageous in this procedure that a heat exchanger or reboiler which serves for heating the bottom of the stripping column is required. This is exposed to high thermal stresses, in particular in non-steady-state conditions, such as starting up the stripping process or plant stoppage. If there is a failure of this component, this generally leads to a complete plant shutdown, which can extend over an unacceptable long period, up to some weeks.

It is an object of the present invention to specify a method of the type in question for separating off an N₂-rich fraction from a liquefied hydrocarbon-rich fraction, which method avoids the abovementioned disadvantages.

To achieve this object, a method of the type in question for separating off an N₂-rich fraction from a liquefied hydrocarbon-rich fraction is proposed, which is characterized in that a first substream of the liquefied hydrocarbon-rich fraction is applied to the stripping column as reflux, whereas a second substream of the liquefied hydrocarbon-rich fraction is fed to the bottom of the stripping column, wherein the second substream of the liquefied hydrocarbon-rich fraction has a higher temperature than the first substream of the liquefied hydrocarbon-rich fraction at liquefaction pressure.

According to the invention, henceforth a substream of the liquefied hydrocarbon-rich fraction itself serves as bottom heating for the stripping column. This makes the provision of an additional heat exchanger or reboiler, as is required in the prior art, superfluous. All of the disadvantages associated with this component are consequently avoided in the method according to the invention. However, it is disadvantageous in the method according to the invention that the energy consumption of the liquefaction process selected rises slightly.

Further advantageous embodiments of the method according to the invention for separating off an N₂-rich fraction from a liquefied hydrocarbon-rich fraction which are subject matter of the dependent claims are characterized in that

-   -   the first substream and/or the second substream of the liquefied         hydrocarbon-rich fraction, before being fed into the stripping         column is or are subjected to an expansion, preferably an         expansion in a liquid expander,     -   the temperature difference between the first substream and the         second substream of the liquefied hydrocarbon-rich fraction         before expansion to the pressure of the stripping column is         between 40 and 100° C., preferably between 60 and 80° C.,     -   the first substream of the liquefied hydrocarbon-rich fraction         which is applied to the stripping column as reflux has a gas         fraction of at most 80% by volume, typically has a vapour         fraction after expansion of 20% by volume, preferably 10% by         volume,     -   the quantitative ratio between the first substream and second         substream of the hydrocarbon-rich fraction which is fed to the         stripping column is variable and     -   in addition to the second substream of the liquefied         hydrocarbon-rich fraction, at least one further methane-rich or         nitrogen-rich stream is fed to the bottom of the stripping         column.

The method according to the invention for separating off an N₂-rich fraction from a liquefied hydrocarbon-rich fraction and also further embodiments of the same will be described in more detail hereinafter with reference to the example method shown in the FIGURE.

The hydrocarbon-rich stream to be liquefied, hereinafter termed the natural gas (stream), which is at a pressure between 30 and 120 bar, is fed to a liquefaction process via line 1. This liquefaction process is only shown in diagrammatic form in the FIGURE, more precisely in the form of two heat-exchanger regions E1 and E2 and also a black box R which is meant to represent the components of one or more refrigerant cycles and/or mixed refrigerant cycles. The procedure of the invention can in principle be combined with all known liquefaction methods.

In the embodiment of the method according to the invention shown in the FIGURE, in the heat exchanger E1 there proceeds a liquefaction and also if appropriate a slight subcooling of the natural gas stream to be liquefied against a (mixed) refrigerant stream which is conducted via line 4 through the heat exchanger E1. However, if the natural gas is present in supercritical form, this is the case, depending on the composition of the natural gas, from approximately 60 bar, strictly speaking no liquefaction and subcooling any longer proceed, but rather an increase in density by means of cooling.

The liquefied and if appropriate slightly subcooled natural gas stream is taken off from the heat exchanger E1 via line 1′ and divided into two substreams 2 and 3.

The first substream is subcooled in the heat exchanger E2 against the (mixed) refrigerant stream 5 which is conducted through the heat exchanger E2 via line 5, expanded so as to produce cold in the liquid expander X1 and subsequently applied via line 2′ to the stripping column S at the top thereof as reflux. The stripping column S is customarily operated in a pressure range between 1.0 and 2.0 bar, preferably between 1.0 and 1.3 bar.

The second substream of the liquefied and if appropriate slightly subcooled natural gas is fed via line 3 to a liquid expander X2, expanded so as to produce cold in this, and subsequently conducted via line 3′ to the bottom of the stripping column S. The abovementioned expander X2 can if appropriate be constructed as what is termed a two-phase expander in which the fluid is present in two phases at the outlet.

The abovementioned expanders X1 and X2 can be provided optionally. If they are not provided, or if one of these expanders is not provided, generally use is made of expansion valves a and b, by means of which the substreams fed to the stripping column S are expanded to the pressure of the stripping column S, in the lines 2 and 3. Omission of these expansion valves a and b, with simultaneous omission of the abovementioned expanders X1 and X2, would be conceivable if the substreams fed to the stripping column S by the lines 2 and 3 were already present at the pressure prevailing in the stripping column S.

The temperature difference between the first substream and second substream of the natural gas stream fed to the stripping column S is, before expansion thereof, between 40 and 100° C., preferably between 60 and 80° C.

The parameters pressure and temperature of the second natural gas substream fed to the stripping column S in the bottom thereof must be selected such that this substream, after its expansion to the pressure of the stripping column S, is present in two phases. As a result, the gas phase can be used as stripping vapour for the stripping column S.

Via the control of the quantitative ratio between the two natural gas substreams 2/2′ and 3/3′ and also of their vapour fractions after their expansion to the pressure of the stripping column S, the desired nitrogen stripping from the LNG product produced in the bottom of the stripping column S can be achieved. At the same time, by means of this procedure, as previously, the methane content of the overhead product of the stripping column S can be controlled or set.

The LNG product is taken off from the bottom of the stripping column S via line 6 and fed, for example, to an LNG store T. From this it can be fed by means of the pump P via line 7 to an LNG carrier C.

The nitrogen-rich fraction taken off at the top of the stripping column S via line 9 is customarily compressed in one or more stages to the desired delivery pressure and fed via line 10 to its further use, for example as combustion gas.

The compression V is preferably implemented by means of a cold-intake compressor. The use of a cold-intake compressor enables the pressure in the stripping column S to be close to atmospheric pressure without having to risk oxygen ingress into the nitrogen-rich fraction which is taken off via line 9, which would be feared in the event of warming of the nitrogen-rich fraction which is taken off via line 9 owing to a pressure loss.

Advantageously, the compressor V which is used for the compression has adjustable inlet blades. These make possible a high load range without a requirement of gas recycling which can be achieved via line 11. The nitrogen-rich fraction fed via line 11 to the bottom of the stripping column S is what is termed the surge protection fraction.

The boil-off gas produced during loading of the LNG carrier C can likewise be fed via line 12 to the bottom of the stripping column S and/or be recycled via the lines 12 and 8 to the LNG store T. In addition, the boil-off gas produced in the LNG store T can be fed via the lines 8 and 12 to the bottom of the stripping column S. Via this means, also the abovementioned fractions fed to the bottom of the stripping column S, together with the second substream of the liquefied natural gas stream, can serve for the stripping process.

Customarily, in line 8, a fan G, which is shown dashed in the FIGURE, must be provided, by means of which the boil-off gas produced in the LNG store T is fed to the bottom of the stripping column S. Omission of such a fan is possible when the stripping column S can be mounted sufficiently high above the LNG store T, so that although the pressure in the LNG store T can be higher than in the bottom of the stripping column S, the provision of a pump in line 6 is not necessary. 

1. A method of separating off an N₂-rich fraction from a feed of a nitrogen-containing liquefied hydrocarbon-rich fraction, wherein the liquefied hydrocarbon-rich fraction, after its liquefaction and subcooling, is fed to a stripping column having a top section and a bottom section, which serves to separate off the N₂-rich fraction, characterized in that the feed nitrogen-containing liquefied hydrocarbon net fraction is split into at least 2 substreams, a first substream (2, 2′) is passed to a top section of a stripping column (S) as reflux, and a second substream (3, 3′) is fed to a bottom section of the stripping column (S), wherein the second substream of the liquefied hydrocarbon-rich fraction (3, 3′) has a higher temperature than the first substream of the liquefied hydrocarbon-rich fraction (2, 2′).
 2. A method according to claim 1, characterized in that the first (2, 2′) substream and/or the second substream of the liquefied hydrocarbon-rich fraction (3, 3′), before being fed into the stripping column (S) is or are subjected to expansion (a, b, X1, X2).
 3. A method according to claim 1, characterized in that prior to expansion the temperature difference between the first (2, 2′) substream and the second substream of the liquefied hydrocarbon-rich fraction (3, 3′) is between 40 and 100° C.
 4. A method according to claim 2, characterized in that the first substream of the liquefied hydrocarbon-rich fraction (2, 2′) which is passed to the stripping column (S) as reflux has a gas fraction of at most 80% by volume.
 5. A method according to claim 1, characterized in that the quantitative ratio between the first substream and second substream of the hydrocarbon-rich fraction (2, 2′, 3, 3′) which is fed to the stripping column (S) is variable.
 6. A method according to claim 1, characterized in that, in addition to the second substream of the liquefied hydrocarbon-rich fraction (3, 3′), at least one further methane-rich or nitrogen-rich stream (8, 11, 12) is fed to the bottom section of the stripping column (S).
 7. A method according to claim 3, wherein the temperature difference is between 60° and 80° C.
 8. A method according to claim 1, wherein the feed is natural gas.
 9. A method according to claim 4, wherein the reflux after expansion has a vapor fraction of 10-20% by volume. 